Conventional mass soldering of printed wiring boards is of many types. Wiring boards with the circuit components inserted are sometimes floated on a bed of molten solder in a heated tank. Molten solder is sometimes made to form one or more waves, and the board is passed over these waves. The molten solder can also be pumped at a substantial pressure to play in streams onto the bottom of a printed wiring board as it passes over a series of solder nozzles.
Mass soldering by these techniques has certain advantages and disadvantages. It is relatively inexpensive; however, solder comprises a mixture of tin and lead in proportions suitable for the particular soldering job intended, and, at the elevated temperatures of soldering operations, tin and lead have a tendency to oxidize in the atmosphere. The oxides of tin and lead are generally lighter than the unoxidized metals and tend to float on top of the molten mass of solder with other impurities. This floating matter is referred to by the general term "dross." When the bottom of the circuit board touches the top of the surface of solder, the solder dross may become a part of the soldering mechanism. This is very detrimental to the quality of the resulting solder joint. Dross tends to impede wetting and also causes pin-holes and other imperfections in the solder.
When the printed wiring board separates from the hot solder, drops of molten solder tend to hang down as projections or stalactites of solder descending from the printed wiring pattern and generally from wires or terminal pins or posts that extend through the printed circuit wiring board. These projections, known as icicles when solidified, extend generally in a direction perpendicular to the plane of the board and are generally pointed so as to constitute a safety hazard to anyone subsequently handling the board. If not removed, a solder projection may break off inside an equipment cabinet and may short or otherwise interfere with the circuits in large, complex installations.
If wire-wrap binding posts or terminals are mounted on the wiring side of the board, these wiring posts also get a coating of solder when soldered by conventional mass-soldering techniques. This is generally not desirable since the binding posts are sometimes lightly gold plated for good electrical contact, and a secure connection by the wire-wrap technique depends upon the sharp corners of the square post biting into the surface of the wire as it is wrapped about the post. Soft solder on the surface of a post tends to oxidize and impede good contact as well as cushion the sharp edges and limit the desired biting necessary for a good connection. If a solder-free portion is to be maintained on the binding posts, it must be masked with a solder resist; and the resist must be removed after soldering. These additional steps of masking and mask removal undesirably add cost to circuit manufacture.
When wiring patterns are provided on both sides of an insulated substrate or even in several layers throughout the thickness of the board, the electronic components are generally mounted on only one side of the board. Reliable connections from the lands of the wiring pattern on one surface of the board to the lands of another wiring pattern on another surface of the board are usually made through the holes into which component leads are inserted. Such connections are also often made by special rivets, braids or other connectors inserted in the board. Using special, inserted interconnect devices is very costly of material, labor and machine time.
Alternatively, copper is plated onto the insides of the holes linking the two or more printed wiring patterns. Typically, a thin coating of copper is electroless plated onto the entire surface of the perforated or drilled, copper-clad laminate including the insides of the holes. More copper is then electroplated all over the board including through the holes. Solder is then selectively plated in the desired circuit pattern or uniformly plated and selectively removed so as to leave an etch resist of solder in a circuit pattern on both of the exposed copper surfaces and on the plated copper that extends through the holes in the board. The board is then etched to remove the exposed copper and leave the solder-plated wiring pattern on both sides of the board plus the through-hole plated-copper interconnections. The leads of the circuit components are then inserted through the holes, and the board is subsequently mass soldered.
In the case of a multi-layer board, each intermediate circuit at a different stratum between the two surfaces of the board can make contact with the plated-copper through-hole connection. A circuit makes contact with the plated-copper through-hole connection by extending to the edge of the hole so the edge of the copper of the circuit forms part of the surface upon which the through-hole copper is plated. This through-hole plating technique involves a rather expensive two-step copper plating process in the manufacture of printed wiring boards.
When soldered by conventional mass soldering methods, the various through-hole connections depend upon capillary action to draw molten solder along the component lead from one side of the board to the other. This technique is nominally free of additional cost in a mass-soldering operation. However, plated through-hole connections are notorious for gaps or cracks in the copper cylinder that extends from one side of the board to the other, through the hole. Therefore, it is standard practice in the electronics manufacturing industry to require a solder fillet on the upper side of the board. A solder fillet on the top of the board is an indication that solder extends all the way through the hole and has bridged any cracks or gaps in the through-hole plating -- thereby assuring good electrical continuity. Full solder flow through the length of the hole also assures good physical strength, in the case of a wire-wrap terminal post, to withstand the strains of the wire-wrap operation.
However, to draw solder from the bottom surface of the board so as to form a solder fillet with the copper on the upper surface requires a suitable capillary clearance or gap. If the maximum capillary clearance is less than about 0.003 inch, it has generally been found that flux vapors do not readily escape. This causes holes and voids in the solder. If the maximum capillary clearance is greater than about 0.005 inch, the solder is less likely to rise through the hole in the board since the wider the capillary clearance, the weaker the overall capillary lifting capability. With a range of ideal capillary clearance of from 0.003 to 0.005 inch, manufacturing tolerances on the pin or post and the plated-through hole become undesirably restrictive and thus quite costly.
In an effort to allow greater tolerance and greater clearance, tiny doughnut-shaped preformed rings of solid solder are manually or automatically placed over the wire and atop the desired soldering site. These preforms are then melted in situ by radiation, vapor condensation, or hot gas convection to flow down into the aperture in the wiring board. However, use of preforms entails a high price in material and labor or automatic machinery to put the relative expensive solid preforms into place prior to melting them. However, with preforms on top of the holes, the clearance can be made substantially larger than the ideal capillary clearance with resultant loosening of tolerances for the sizes of holes and pins, posts, terminals, or wire.